1. Field of the Invention
The present invention relates to a digital audio system and, more particularly, to a signal processor for providing both sub-band coding and decoding in a digital audio system.
Korean Patent Application No. 93-5900 is incorporated herein by reference for all purposes.
2. Brief Discussion of Related Art
FIG. 1 shows an encoder for performing data encoding in a conventional universal sub-band coding system developed through a joint effort by the CCETT in France, the IRT in Germany and Philips in the Netherlands. The encoding block of the sub-band coding system is composed of a scale sample generator 10, a quantizer 11, a scale factor generator 12 and a formatter 13.
The operation of the sub-band coding system utilizing the configuration discussed above will now be briefly described.
It will be noted that each of 32 sub-bands of a frame signal is represented by 12 samples, thus 384 sub-band samples are included in each frame. The sample with the highest amplitude of each sub-band is used to determine a scale factor for every respective sub-band. Then, the samples are scaled by the respective scale factor in order for them to be normalized with respect to the value representing the highest amplitude.
One part of the encoding algorithm will have to supply an allocation information table, e.g., information regarding word length for each sub-band in the frame. It will be noted that it is possible to generate the allocation information table in various ways, the simplest way being to use the allocation information table in which all entries are fixed. However, this information may also change with every frame.
The samples are then quantized to reduce the number of bits of the samples to the number specified for the sub-band in the allocation information table and, then, the synchronization and system information is added to produce a formatted frame.
FIG. 2 shows a decoding block of a general sub-band coding system employed in the decoder according to the CCETT/IRT/Philips technique. The decoding block of the sub-band coding system includes a deformatter 20, an allocation information RAM 21, a scale factor generator 22, a dequantizer 23, a multiplier 24, a decoder controller 25, a multiplication factor generator 26 and a demultiplexer 27.
The decoding operation of the sub-band coding system according to the above configuration will now be described. It will be noted that the decoding process is always performed in the same manner. The essential information is decoded using the scale factor and the sub-band samples. All other information is included so as to permit the decoding to be performed correctly. The decoding process is repeated for every frame. After the reception of the synchronization and system information, the allocation information is used in storing the scale factor array correctly in generator 22.
The scale factor array is an array with two columns, 6 bits wide, and 32 rows. Each column corresponds to one of the output channels and each row corresponds to one of the 32 sub-bands. The received scale factors are stored in this array. The location of a scale factor which has not been transferred becomes 1111111, indicating that the output samples for that sub-band are all "0".
In stereo and bilingual modes, the scale factors for the two channels are stored in the two columns of the array. In mono mode, the same scale factor is stored in both columns of the array.
It will be noted that the scale factors are used as an index into a fixed table. This table contains the multiplication factors by which the sub-band samples have to be multiplied before they are sent to join transform implemented in the encoder. These scale factors are then used for both encoding and decoding operations.
In other words, the encoding block or circuit is independent of and separated from the decoding block or circuit in the conventional general sub-band coding system, making the configuration of the player/recorder very complex.